APPSC AEE (Civil/Mechanical) 2012
⭕ (a) the boundary surface is relatively smooth
⭕ (b) the roughness projections are of low height
⭕ (c) the roughness elements are completely covered by the laminar sub-layer
⭕ (d) the laminar layer is thin as compared to the average height of roughness elements
Explanation: Pipes are considered hydraulically smooth when the roughness elements are entirely submerged within the laminar sub-layer. In such conditions, the roughness does not interfere with the turbulent core, and behaves as if the pipe wall is smooth. This is typical in steel and cast iron pipes carrying fluid at high Reynolds number with minimal frictional resistance due to wall roughness.
Hydraulically Smooth Pipes in Fluid Mechanics
When analyzing flow in pipes, surface roughness plays an important role in determining resistance. A pipe is termed hydraulically smooth if roughness does not affect the flow significantly, meaning it lies within the laminar sublayer of the flow profile.
Key Concepts:
- The laminar sub-layer forms near the pipe wall in turbulent flow.
- If roughness elements are shorter than this layer, they do not disrupt the flow.
- Such pipes behave like smooth pipes in terms of friction factor.
Conclusion:
The correct answer is (c) the roughness elements are completely covered by the laminar sub-layer.
